linux-kernel - Re: [PATCH] tracing: Fix filter string testing

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Message-ID: <20250417203438.413da315@batman.local.home>
Date: Thu, 17 Apr 2025 20:34:38 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Andrii Nakryiko <andrii.nakryiko@...il.com>, Mykyta Yatsenko
 <mykyta.yatsenko5@...il.com>
Subject: Re: [PATCH] tracing: Fix filter string testing

On Thu, 17 Apr 2025 16:29:27 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Thu, 17 Apr 2025 18:30:03 -0400 Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > From: Steven Rostedt <rostedt@...dmis.org>
> > 
> > The filter string testing uses strncpy_from_kernel/user_nofault() to
> > retrieve the string to test the filter against. The if() statement was
> > incorrect as it considered 0 as a fault, when it is only negative that it
> > faulted.  
> 
> changelog forgot to describe the userspace-visible effects of the bug?
> 
> > Cc: stable@...r.kernel.org  
> 
> Which is more important when proposing this!
> 

I can update the change log to show how it's broken. In fact, I'm
working on a selftest to catch it if it breaks again.

  # cd /sys/kernel/tracing
  # echo "filename.ustring ~ \"/proc*\"" > events/syscalls/sys_enter_openat/filter
  # ls /proc/$$/maps
  # cat trace

If it works you get:

              ls-1192    [007] .....  8169.828333: sys_openat(dfd: ffffffffffffff9c, filename: 7efc18359904, flags: 80000, mode: 0)

If not, you get nothing!

-- Steve